A linear motor is an electromagnetic machine in which electrical current through a coil winding carried by one part of a motor interacts with a magnetic flux emanating from another part of the motor to create a relatively high level of force between the two motor parts in a direction normal to the coil current and magnetic flux. The two parts of the motor are arranged to allow the occurrence of a limited linear displacement between the parts in reaction to the force created. Reciprocation between the parts may be obtained by periodically reversing the direction of current flow through the coil. The force of displacement created between the two parts of the motor may be harnessed by holding one of the parts stationary and connecting the other part to pulse the cone of a loud speaker, for example, or to drive the piston of a vibrator.
The amount of work obtainable from a reciprocating linear motor is proportional to three variables: namely the amplitude of the displacement force created between the moving and stationary parts, the length or "stroke" of the linear displacement between the parts, and the frequency with which linear displacement occurs. The work obtainable may be maximized, therefore, by maintaining a large force between the moving and stationary parts during each stroke, by designing the moving part to traverse the longest possible distance during each stroke, and by operating the motor to cause the strokes to occur with the greatest frequency. Particular design considerations may restrict one of these variables. An effort to miniaturize a motor, for example, may require shortening of its stroke, thereby necessitating operation of the motor at a higher frequency to provide the same amount of work. Other considerations may necessitate a motor able to perform variable amounts of work by operating over a wide band of frequencies.
Previous efforts to maximize the work obtainable from reciprocating linear motors have included proposals of elaborate electrical networks for increasing the displacement force between the movable and stationary parts by controlling the instantaneous current flow through the coil winding. Such networks, which usually involve various arrangements of coils and taps, are complex and are prone to failure due to the sheer number of component parts.
Maximization of obtainable work is not the only consideration influencing the design of a linear motor. Smooth and efficient operation mandates that the displacement force created between the motor parts be nearly linearly proportional to the instantaneous coil current throughout the length of each stroke. The ratio of the displacement force to coil current is a term called the "force constant." A motor exhibiting a displacement force that varies linearly with coil current will have an unvarying force constant over the length of a stroke. This characteristic is not exhibited by previously proposed linear motors where coil windings have axial lengths equal to the axial lengths of the sources of magnetic flux. They are also lacking in the symmetry advantage inherent in having two magnet rings.
Other previous proposals have attempted to provide smooth and efficient linear motor operation while maximizing the work attainable by improving various mechanical characteristics of a motor. One proposal sought to provide a reciprocating linear motor in which the force exerted on the moving part remained constant throughout substantially the entire length of its stroke by using a single, radially polarized ring magnet which serves as the moving part (i.e., the "armature") of the motor. Pairs of taps simultaneously sliding along an outer coil wound inside a U-shaped, cylindrical stator core follow the armature travel. The sliding taps control the distribution of current through the outer coil and through an inner coil wound around a stationary inner core joined by a central end section to the outer core and keeps the equal current flowing through the coil sections on either side of the armature. This configuration of pairs of sliding taps and outer and inner coils fails to provide a linear motor having an unvarying force constant, over the length of a stroke, thereby creating a unidirectional restoring force along the longitudinal motor axis which has an amplitude dependent upon the location of the armature magnet. This restoring force acts as a bias opposing displacement of the armature away from the central core section. Additionally, the continuous sliding action of the taps presents a source of wear likely to cause early failure of the motor. Moreover, the use of a single ring magnet in the armature presents a source of radial instability due to such influences as magnetic attraction between the magnet and the cores as well as an intrinsic axial bias force if the current is not distributed through the coils with precise equality on both sides of the armature.